1. Field of the Invention
The present invention relates to a solid-state image pickup device, and in particular, to a solid-state image pickup device which includes photosensitive cells formed in a laminated layer of a photoconductive material such as amorphous semiconductor, and imaging apparatus using such a device.
2. Description of the Prior Art
A solid-state image pickup device does not possess an extensive dynamic range where an output current therefrom is proportional to the intensity of a light incident to the image pickup device. This restriction causes a problem when taking pictures of various objects under differing conditions.
In a microfilm reader for projecting a picture of a frame recorded on microfilm onto a screen in a magnified scale, for example, if a solid-state image pickup device is utilized, complex and sophisticated signal processing electronics are required to obtain video signals most suitable for displaying the picture. In general, a silver-halide photosensitive film, a diazo-compound photosensitive film, or the like may be used for a microfilm. Usually, most silver-halide films have negative images recorded thereon, whereas diazo-compound films contain positive images obtained by copying images carried on a silver-halide film placed fixedly on the diazo-compound film in ordinary cases. Consequently, microfilms contain either negative or positive images. In addition, pictures recorded on microfilm may be of various gradations, for example, of a text on which characters and other symbols are written, and a picture having intermediate gray scale gradations.
In order to shoot images recorded on a microfilm, containing such various pictures as described above with a solid-state imaging device to obtain video signals suitable for display, a signal processing circuit is required which includes contrast conversion of images from hard to soft, or vice versa, as well as inversion of images from negative to positive, or vice versa (positive/negative conversion). This signal processing necessitates complicated circuits for performing a variable gamma (.gamma.) correction to be conducted on a signal delivered from the image pickup device, for inverting the level of the output signal, and for executing associated operations.
Since spectral transmission characteristics are different between a silver-halide film and a diazo-compound film, the spectral sensitivity characteristics of the solid-state image pickup device must be variable to obtain video signals suitable for display.
In addition, it has been desired, when transmitting manuscripts or printed matter having various colors by such a transmitter device as a facsimile, to emphasize characters written in red or to copy a blue background into a white portion background, for example. These requirements can be satisfied with an image pickup device which has variable spectral sensitivity characteristics.
On the other hand, a solid-state image pickup device having photosensitive cells formed by a photosensitive material such as an amorphous semiconductor or the like in a layer-built structure has a large numerical aperture with respect to an incident light, and thus has a high sensitivity, so that the degree of freedom in designing the spectral sensitivity could be increased and the frequency range in which the device is sensitive could be quite wide. Moreover, the blooming phenomena can be effectively suppressed. The present invention accordingly facilitates construction of a camera having variable gradation characteristics covering the operation on the contrast of output video signals and the conversion between the negative and positive images, and having variable spectral sensitivity characteristics based on the characteristics of the image pickup device described above.
The words "output characteristics" should be appreciated in the broadest sense, that is, they also imply those associated with the negative/positive conversion and spectral sensitivity in addition to the difference in the optical density alone.